Mitochondrial Analysis of Gene Flow between New Zealand Mallards (Anas platyrhynchos) and Grey Ducks (A. superciliosa)

One of the more well-known examples of hybridization in birds is the frequently documented occurrence between sexually dimorphic Mallards (Anas platyrhynchos) and several closely related nondimorphic species in the mallard complex. In New Zealand, the Grey Duck (Anas superciliosa superciliosa) is the indigenous, nondimorphic Mallardlike species, and extensive hybridization with introduced Mallards has been implicated in the population decline of Grey Ducks. Individuals from throughout the country were classified phenotypically as parentals or hybrids based on variation in plumage, bill color, and leg color. We confirmed species-specific mitochondrial DNA haplotypes by comparing restriction-enzyme fragment patterns in Grey Ducks and New Zealand Mallards to those of Pacific Black Ducks (A. superciliosa rogersi) from Australia and Mallards from North America, respectively. Our data indicate that hybridization has led not only to introgression of Grey Duck mtDNA into Mallard populations (the predicted direction of gene flow), but also to significant introgression of Mallard mtDNA into Grey Duck populations. Thus, the contention that hybridization between Mallards and nondimorphic species involves primarily Mallard males with females of the other species is not upheld for this example from New Zealand. The speciation process appears to be undergoing reversal.